I think it would be a good idea to establish the rules of the zombies. What can they do? What can't they do? What advantages does the player have? What are the player's disadvantages? If you are planning on making a relatively open world game then it would make sense to have the zombie's behavior tree include stopping to eat something. But IMHO I think it would be better suited to make it a slaughter fest where the player has many options but can become overrun very quickly if they aren't quick on their feet. You could even have a mechanic where the player comes up to a singular zombie and they have a choice, do I silently kill the zombie or do I shoot them? If the player decides to shoot then it will attract more zombies to their location. On the other hand you could make close combat relatively risky for the player. Just some thoughts, keep up the amazing work brother!!!